1. Field of the Invention
The present invention relates to a humidification control method for a fuel cell formed by stacking an electrolyte electrode assembly and a separator. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as an electrolyte membrane (electrolyte). The polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a fuel cell. A plurality of fuel cells are stacked together to form a fuel cell stack, e.g., mounted in a vehicle.
In the fuel cell, in order to perform the reliable power generation, it is required to correctly keep track of the power generation condition. For example, the electrolyte membrane needs to be humidified to achieve the desired humidified state for maintaining the power generation performance. If the electrolyte membrane is dried, the power generation performance is degraded.
If the quantity of the water produced in power generation is large, and the water is excessively present in the fuel cell, flooding may occur undesirably. Therefore, clogging occurs in channels for reactant gases to degrade the power generation performance. Further, in some cases, the power generation performance may be degraded due to the insufficient supply of the fuel gas.
In this regard, the quantity of water in the fuel cell can be measured by measuring impedance of the fuel cell. For example, an alternating current impedance method has been adopted. In this method, the impedance is measured at a large number of points while changing the applied sinusoidal frequency.
However, in the alternating impedance method of this type, it takes a considerable time to perform one measurement. Therefore, it is difficult to detect the state of water content (humidified state) within the fuel cell in real time, disadvantageously.
In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2003-086220 is known. The fuel cell system includes a fuel cell capable of obtaining electrical energy generated by electrochemical reactions of hydrogen and oxygen. In a case where a sinusoidal wave signal is applied to the output signal of the fuel cell while changing the frequency, complex number impedance of the fuel cell is used to obtain resistance component which increases when shortage of the water quantity within the fuel cell occurs and the resistance component which increases when the water quantity within the fuel cell is excessive. Based on the resistance components, the state of water content within the fuel cell is estimated.
The fuel cell system includes a sinusoidal wave applying means for applying a sinusoidal wave signal having an arbitral frequency to an output signal of the fuel cell, voltage detection means for detecting the output voltage of the fuel cell, current detection means for detecting the output current of the fuel cell, and impedance computation means. In a case where a sinusoidal wave signal at a predetermined frequency is applied to the output signal of the fuel cell by the sinusoidal wave applying means, complex number impedance of the fuel cell at the predetermined frequency is calculated based on the output voltage detected by the voltage detection means and the output current detected by the current detection means.
The impedance computation means calculates the complex number impedance of the fuel cell at one frequency at least, and based on the complex number impedance of the fuel cell at the at least one frequency, calculates the complex number impedance of the fuel cell in a case where a sinusoidal wave signal is applied while changing the frequency.